How to install rotobezier in blender
As noted in the link on @DuarteFarrajotaRamos' comment.

Rotobezier is part of blender now: It is no longer an addon and it does not have that name any more.

The functionality is part of how masks work.

To make dynamic masking, all that is needed is to create a mask and create keyframes for the control points. Also the control points for masks can be controlled using tracking points in the Movie Clip Editor.